DESCRIPTION (adapted from applicant's Description): A cohort of 133,000 California school teachers has been established by a collaborative group of epidemiological investigators with the goals of evaluating unresolved issues related to breast cancer risk factors and studying other important issues related to women's health. The teachers were recruited with a detailed multiple choice, optically-scanned mail survey. Scanning of the questionnaires has been completed and data editing is ongoing. Planned follow-up includes routine linkage with the California Cancer Registry and California mortality files, annual re-contact of cohort members for follow-up, and biennial contact for collecting additional risk factor exposure data and information on other health outcomes. The Specific Aims for this project are to: 1) test a series of unresolved and emerging hypotheses related to breast cancer aetiology (specifically associations with the lactation, hormone replacement therapy, abortion/miscarriage, dietary phytoestrogens, fibre, micronutrient consumption, alcohol intake, physical exercise and activities, family history of breast and other cancers, and active and passive cigarette smoke exposure); 2) conduct calibration/validation studies of the food-frequency questionnaire and self-reported information on family history of breast and other cancers reported in the baseline questionnaire; and 3) follow this cohort for five additional years, during which time, two or more questionnaires will be mailed to update initial exposure assessments, collect new exposure information, and assess additional disease outcomes for testing novel hypotheses of major importance to women's health, in a timely manner. During the next five years, 2,025 invasive incident and 390 in situ incident breast cancers are anticipated which will provide ample statistical power to address each of the proposed hypotheses in detail. The California Teachers Study presents a rare opportunity to study women's health, because of the size of the cohort, the uniformly high level of education among teachers, their experience with survey instruments, their diversity of exposures and geographic residences, and the relative ease with which they can be followed in California. This research is intended to substantially increase knowledge of preventable risk factors for cancer and other health outcomes.